The high-resolution He 584Aophotoelectron spectra of heptafluoroquinoline and heptafluoroisoquinoline are compared with those of the parent compounds. Shifts in π ionisation potentials, due to the fluorine substitution, can be described with an inductive and a combined inductive-conjugative Hu¨ckel model

Ham, D.M.W. van den

Meer, D. van der

Chemical Physics Letters

English

van den Ham, D.M.W.

van der Meer, D.

University of Twente Research Information

Volume 15, number 4 CHEMICAL PHYSICS LETTERS 1 September 1972 P E R F L U O R O  E F F E C T  IN T H E  P H O T O E L E C T R O N  S P E C T R A  O F  Q U I N O L I N E  A N D  I S O Q U I N O L I N E  D.M.W. VAN DEN HAM and D. VAN DER MEER Chemical Physics Laboratory, Twente University of Technology, Enschede, The Netherlands Received 17 May 1972 The high-resolution He 584 A photoelectron spectra of heptafluoroquinoline and heptafluoroisoquinoline are compared with those of the parent compounds. Shifts in ~r ionisation potentials, due to the fluorine substitution, can be described with an inductive and a combined inductive-conjugative Hiickel model. I. Introduct ion 3. Discussion In the photoelectron spectra of  aza- and diaza- naphthalenes [ 1, 2] ,  one can distinguish between bands due to ionisation from rr, a and n (lone-pair) molecular orbitals (MO's). Quite often n bands in these spectra are obscured by partial or total  overlap with zr bands. To separate these rr and n bands one can try to make use of  the so-called "perf luoro effect". Brundle et al. [3, 4] have shown that substi tution of  hydrogen atoms by  fluorine in aromatics, influences the o and rr MO's to a different extent.  As "lone-pair"  electrons in aza aromatics are delocalised to some degree, one also expects a different behaviour of  n and zr MO's on fluorination. 2. Method and results The spectra were recorded as described previously* [2] and are presented in figs. 1 and 2. The measured ionisation potentials are collected in table 1. Although the photoelectron spectra of  quinoline and isoquinoline have been published before [5],  we present them again for reasons of  comparison. * Perkin-Elmer, model PS 15, borrowed from Dr. D.W. Turner. 3.1. Analysis o f  the spectra The analysis of  bands in the spectra of  naphthalene, quinoline and isoquinoline has been given by Eland and Danby [5] on the basis of  HMO calculations. The analysis of  naphthalene has been confirmed by Brundle et al. [4] with the aid of  the perfluoro effect. It has been observed [1, 2] that in t roduct ion of  nitrogen atoms in the aromatic nucleus does not  alter the shape of  the n bands. The same phenomenon ap- pears in the perfluoro series. By comparing the spectra of heptafluoroquinoline (A) and heptafluoroiso- quinoline (B) with that of  octafluoronaphthalene (C) [4] one can easily discern the n bands. From 13.5 eV on, the spectra become undecipherable due to the host  of  o ionisations and ionisation from the fluorine lone pair, which can appear from 14 eV on. In the spectrum of (B)  we find an extra band at 11.6 eV, close to the third 7r band. As such a band does not appear in the spectrum of  (C), this must be at tr ibuted to an ionisation from an n MO. In (A) no extra band is found, although the third n band has an abnormal intensity compared to the same band in (B) and (C). Therefore we presume that here the n band and the third n band overlap. In both cases the n band shifted ca. 2.2 eV to higher ionisation po- 549 Volume 15, number 4 CHEMICAL PHYSICS LETTERS 1 September 1972 "r~,rl / ~J / / : / ,/ J rl: n :'n 15 IONISATION FOTENTIAL (e.'d) Fig. 1. Photoelectron spectra of quinoline and heptafluoro- quinoline. tentials compared to the parent compounds. 3.2. Correlation o f  ~r ionisation potentials with HMO calculations To calculate the effect of  fluorine substitution on 7r MO's with the HiJckel method,  we can choose an in- ductive (model 1) or a conjugative model (model 2). For model 1 we took the parameters CtC(F) = aC + 0.3 ~CC (Streitwieser [6] ), a N = a C + 0.7/3CC [1] and/3CC = ~CN- The ionisation potentials calculated in this way, correlate well with the experimental values (table 1, fig. 3). For model 2 Streitwieser [6] proposes the param- eters ot F = ot C + 3.0/3CC and/~CF = 0.7/3CC. Fig. 3 shows that the relative positions of  the 7r ionisation potentials in the perfluoro series are predicted well, but that they are not in accordance with those cal- culated for the parent compounds. To improve the overall correlation we introduced an inductive effect in the conjugative model (model 3), " l r n  , L , ' ; '  ,: i i' r J IF i : r I F j i j i I I I jF F tO 15]~NJ~ATION POTENTIAL (e V )  Fig. 2. Photoelectron spectra of isoquinoline and heptafluoro- isoquinoline. with the parameters a F = ct C + 3.0 flCC,/3CF = 0.7 flCC and aC(F) = a C + 0.6/3CC. The results are presented in table 1 and fig. 3. They show that fluorine substitution influences 7r, n and o ionisation potentials to a different extent. We intend to use this perfluoro effect for a more definite analysis of  the diazanaphthalenes. Work on these com- pounds is in progress. Correlations between shifts of  n bands and predictions by all valence electron cal- culations have been postponed until then. Acknowledgement We want to thank Dr. R.D. Chambers (Durham, England) for his generous gift of  the fluorinated com- pounds and Dr. D.W. Turner (Physical Chemistry Laboratory,  Oxford) for the opportunity he gave us to measure the spectra in his laboratory. 550 Table 1 Experimental  and calculated n ionisation potentials.  Exper imenta l  values are in eV, calculated values are in units/3. The values between brackets  are est imated Compound  Orbital Vertical IP IP (calculated) IP (calculated) IP (calculated) (observed) model  1 model  2 model  3 naphthalene [4] n 1 8.13 0.62 0.62 0.62 n2 8.90 1.00 1.00 1.00 n3 10.01 1.30 1.30 1.30 7r4 10.90 1.62 1.62 1.62 quinoline 7r I 8.62 0.73 0.73 0.73 ~r2 (9.16) 1.00 1.00 1.00 7r3 10.58 1.41 1.41 1.41 n (9.2) isoquinoline 7r 1 8.54 0.65 0.65 0.65 n2 (9.40) 1.10 1.10 1.10 7r3 10.42 1.33 1,33 1.33 n (9.40) octafluoro- n ]  9.05 0.92 0.43 0.98 naphthalene [4] lr 2 10.08 1.19 0.85 1.21 n3 10.96 1.54 1.11 1.51 7r4 11.76 1.92 1.33 1.81 heptafluoro- n I 9.51 0.99 0.56 1.03 quinoline rr 2 10.19 1.20 0.85 1.21 ~r 3 (11.45) 1.61 1.22 1.58 n4 12.22 1.95 1.47 1.85 n (11.4) heptafluoroiso- 7r 1 9.29 0.94 0.47 0.99 quinoline n2 10.61 1.26 0.96 1.26 7r3 11.23 1.55 1.13 1.53 n4 12.57 2.00 1.60 1.90 n (11.6) ~ 1 :  Q; ,_  / f . 7 ~  ~ "~ 11. - • z LtJ . 4 "  ~la . ~ y  z 10. ~ . ~ @ "  o . r  ~ V &J " '~  0 O. "/ V ~  _ _  OV m o d e l  1 -- ,$" • , O  __ 0 ~ . . . . .  0 A m o d e l  3 i , i , , , , , n , • 5 1.0 -5 2.0 • . un i t s  13 IONISATION P O T E N T I A L  ( ca l cu l a ted )  Fig. 3. Plot of  exper imental  versus calculated n ionisation potentials.  Open  circles correspond to the parent  compounds .  551  Volume 15, number 4 CHEMICAL PHYSICS LETTERS 1 September 1972 References [ 1 ] D.M.W. van den Ham and D. van der Meer, Chem. Phys. Letters 12 (1972) 477. [2] F. Broglie, E. Heilbronner and T. Kobayashi, Helv. Chim. Acta 55 (1972) 274. [3] C.R. Brundle, M.B. Robin, N.A. Kuebler and H. Basch, J. Am. Chem. Soc. 94 (1972) 1451. [4] C.R. Brundle, M.B. Robin and N.A. Kuebler, J. Am. Chem. Soc. 94 (1972) 1466. [5] J.H.D. Eland and C.J. Danby, Z. Naturforsch. 23a (1968) 355. [6] A. Streitwieser Jr., Molecular orbital theory (Wiley, New York, 1962). 552  

Perfluoro effect in the photoelectron spectra of quinoline and isoquinoline

D.M.W. van den Ham

D. van der Meer

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Molecular orbital theory

https://research.utwente.nl/en/publications/fb4c5521-e47d-4abc-93af-ec85f7d37e40

Perfluoro effect in the photoelectron spectra of quinoline and isoquinoline

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